Rescuers are working to retrieve a man stuck deep underground inside a cave in the German Alps in an effort that may take days as they negotiate a labyrinth of shafts and bottlenecks.

A four-member rescue team reached the 52-year-old researcher early on Monday (last Monday NZT) inside the Riesending system, near Berchtesgaden in Germany's southeastern corner.

The man, whose name hasn't been released, suffered head injuries a day earlier. One of his two companions made a 12-hour climb back to the entrance to alert authorities, while the other stayed with him.

The injured man is nearly 1km below the surface "in one of the most difficult caves in Europe," mountain rescue official Klemens Reindl told n-tv television.

Officials said he is around 6km from the entrance. He is believed to be in stable condition.

"We have shafts that go straight down 350 metres where you have to rappel down and climb back up on a rope," he said.

The cave system has tight spots where only a slim person can squeeze through, and explorers also have to contend with water.

Rescuers laid a telephone line several hundred metres deep to help the rescue effort, while others set up camps inside the cave system on the border with Austria. They were working in small teams of up to four people each.

Another mountain rescue official, Stefan Schneider, told a televised news conference a doctor was in the cave but it wasn't clear when he would get to the patient.

"It's going round the clock and it's going to last a few more days," Schneider said.

Cave rescue specialists from Germany, Austria and Switzerland are involved in the operation.